Neutrons and Pulsars

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Neutrons and Pulsars

Lab 8
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Neutron Stars

• Neutron stars are the collapsed cores of massive
  stars, 15 to 30 times the mass of our sun
• masses ltlt 15 solar masses the star becomes a
  white dwarf
• masses gtgt 30 solar masses becomes a black hole
• typical mass of a neutron star is 1.4 solar
  masses, and the radius is probably 10 km

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Neutron Stars form when.

• The central part of the star fuses its way to
  iron
• it can't go any farther because at low pressures
  Fe 56 has the highest binding energy per nucleon
  of any element
• which means fusion or fission of Fe 56 will
  require an energy input

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so..

• So the iron core just accumulates until it gets
  to about 1.4 solar masses (the "Chandrasekhar
  mass")
• Then the electron degeneracy pressure that had
  been supporting it against gravity gives up the
  ghost and collapses inward

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then..

• At the very high pressures involved in this
  collapse, it is energetically favorable to
  combine protons and electrons to form neutrons
  neutrinos
• About 1057 neutrinos are made in the iron core,
  as the protons are converted to neutrons
• The neutrinos escape after scattering a bit and
  making more supernovae, and the neutrons settle
  down to become a neutron star, with neutron
  degeneracy managing to oppose gravity

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Whats inside a Neutron Star?

• TOP
• In the atmosphere and upper crust, there are lots
  of nuclei, so it is not primarily all neutrons
  yet
• At the top of the crust, the nuclei are mostly
  Fe56 and lighter elements, but deeper down the
  pressure is high enough that the atomic weights
  rise
• At densities of 106g/cm3 the electrons become
  degenerate, i.e., electrical and thermal
  conductivities are huge because the electrons can
  travel great distances before bumping into each
  other

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The Neutron Drip Layer

• Deeper yet, at a density around 4x1011 g/cm3, is
  the "neutron drip" layer
• At this layer, it becomes energetically favorable
  for neutrons to float out of the nuclei and move
  freely around, so the neutrons "drip" out
• Even further down, mainly free neutrons, with a
  5-10 sprinkling of protons and electrons

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Pasta-Antipasta Layer

• As the density increases, the "pasta-antipasta"
  sequence starts
• At relatively low (about 1012 g/cm3) densities,
  the nucleons are spread out like meatballs that
  are relatively far from each other
• At higher densities, the nucleons merge to form
  spaghetti-like strands, and at even higher
  densities the nucleons look like sheets (such as
  lasagna)
• Increasing the density further brings a reversal
  of this sequence, where there are mainly nucleons
  but holes form (in order of increasing density)
  anti-lasagna, anti-spaghetti, and anti-meatballs
  (also called Swiss cheese)

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Pulsars, Neutron Stars

• Simply put, pulsars are rotating neutron stars.
  And pulsars pulse because they rotate!
• http//www.astro.umd.edu/miller/Images/pulsarSmal
  l2.gif
• rotate very rapidly, up to 600 times per second
• have the strongest magnetic fields in the known
  universe
• center of neutron stars are believed to be 100
  million K

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Spinup and spindowns

• Neutron stars are born rotating fast
• Magnetic field exerts a torque which slows it
  down for ever after
• But glitches can briefly spin it back up again

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Accretion disks

• Stars usually exist as binary systems, so a
  neutron star can accrete from its companion
• If the companion is relatively small, matter
  tends to flow towards the neutron star and forms
  a disk around it
• If companion gt10 solar masses, matter flows
  towards the neutron star as a low angular
  momentum wind

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What happens to the Neutron Star?

• The fate of the hot neutron core depends upon the
  mass of the progenitor star
• If the progenitor mass is 10x mass of the Sun,
  the neutron star core will cool to form a neutron
  star or "pulsars", powerful beacons of radio
  emission
• If the progenitor mass is larger, then the
  resultant core is so heavy that not even nuclear
  forces can resist the pull of gravity and the
  core collapses to form a black hole

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BLACK HOLES!

• Black holes are usually formed when an extremely
  massive neutron star
• A black hole is a region of space in which the
  matter is so compact that nothing can escape from
  it, not even light
• the "surface" of a black hole, inside of which
  nothing can escape, is called an event horizon
• The matter that forms a black hole is crushed out
  of existence
• Just as the Cheshire Cat disappeared and left
  only its smile behind, a black hole represents
  matter that leaves only its gravity behind

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Strange Facts About Black Holes

• Light bends so much near black holes that if you
  were near one and looking away from the hole, you
  would see multiple images of every star in the
  universe, and could actually see the back of your
  own head!
• Inside a black hole the roles of time and radius
  reverse just as now you can't avoid going into
  the future, inside a black hole you can't avoid
  going in to the central singularity
• Singularity in a black hole, the "center point",
  at which densities, tidal forces, and other
  physical quantities become infinite (our current
  physical theories break down at this point)

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Forces at black holes

• Black holes, like any gravitating objects, exert
  a tidal force
• If you approach a black hole feet first, the
  gravitational force at your feet is greater than
  the force at your head
• The tidal force at the event horizon is smaller
  for larger black holes
• You would get torn to shreds far outside a black
  hole the mass of our sun, but at the event
  horizon of a billion solar mass black hole the
  tidal force would only be a millionth of an
  ounce!

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Your friend at a Black Hole

• If you stood a safe distance from a black hole
  and saw a friend fall in, he would appear to slow
  down and almost stop just outside the event
  horizon
• His image would dim very rapidly
• Unfortunately for him, from his point of view he
  would cross the event horizon just fine, and
  would meet his doom at the singularity

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really cool neutron star websites

• http//antwrp.gsfc.nasa.gov/htmltest/rjn_bht.html
• http//cosmology.berkeley.edu/Education/BHfaq.html
  
• http//imagine.gsfc.nasa.gov/docs/science/know_l1/
  pulsars.html
• http//www-astronomy.mps.ohio-state.edu/ryden/ast
  162_5/notes21.html
• http//chandra.harvard.edu/xray_sources/neutron_st
  ars.html
• http//www.herts.ac.uk/astro_ub/a41_ub.html
• http//map.gsfc.nasa.gov/m_uni/uni_101stars.html